The invention relates to a method for triggering a two-stage air bag gas generator in a motor vehicle, and more particularly, to a method in which an acceleration signal is measured, processed and by way of a first evaluation process, a triggering decision is made for triggering the first stage of the gas generator and by way of a second evaluation process, a triggering decision is made for triggering the second stage of the gas generator. The triggering of the second stage is released only after elapse of a defined time period T1 since the ignition of the first stage.
Air bag triggering methods are generally well known. For the evaluation of the seriousness of an accident, for example, threshold values are used which when exceeded by working signals derived from the acceleration signal points to a certain seriousness of an accident. In particular, a method for triggering restraining devices is discussed in EP 0 458 796 B2, in which an acceleration signal is measured and is converted by an integration step into a data processing speed signal which approximately reflects the crash-caused speed reduction. For forming a triggering criterion for an air bag, a threshold value for the mentioned data processing speed signal can be defined. This threshold value, in turn, can be changed as a function of one or several state variables derived from the crash event, for example, as a function of the time elapsed in the crash. Furthermore, it is known to include safety-relevant state variables, such as the buckle switch state, the seat occupation by a person or a child seat, and the occupant position or the occupant weight, in the triggering conditions. The restraining devices are activated as a function of the recognized seriousness of the accident and the occupant condition.
Particularly in the combination of the belt tightener and the air bag, multi-stage restraining systems are known and are used in vehicles, in which, as a function of the seriousness of the accident and the occupant (whether or not the occupant is, for example, wearing a belt), the belt tightener and the air bag is are activated simultaneously, successively or selectively. Staggered triggering thresholds for the belt tightener and the air bag achieve, a triggering action which is graduated according to the seriousness of the accident.
In order to reduce the stressing of the occupant by an inflating air bag, two-stage or multi-stage gas generators are known for filling the air bag. The ignition of the second stage takes place in a time-staggered manner after the ignition of the first stage.
Two-stage gas generators of a first type have two separate gas generating systems with one separate igniter respectively as seen in DE 19541584 A1. A specific quantity of gas is generated in each stage. As only one stage, or both stages are successively ignited in a time-staggered manner, the generated gas quantity flow can be varied for filling the air bag and can be adapted to the seriousness of the accident.
A second type of two-stage gas generators has a single gas generating system with two igniters. After the first ignition, an after ignition can be caused into the expansion phase of the gas by the second igniter in order to accelerate the gas expansion and increase the gas quantity. The effect of the second ignition depends considerably on the point in time of the ignition. At the end of a gas expansion phase initiated by a first ignition, the effect of an after ignition is slight.
Analogously to the initially mentioned method with staggered triggering thresholds for the belt tightener and the air bag, a method for a two-stage gas generator can be employed whereby staggered triggering thresholds are defined for the ignition of the first and second stage. If, because of the evaluation of the acceleration signal, a particularly serious impact with high deceleration values is recognized, both stages are ignited rapidly following one another, which brings the air bag rapidly into a protective condition, as appropriate for the accident situation. However, disadvantages are inherent in this method as explained below.
In order to achieve a sufficiently fast triggering with respect to all relevant crash events, the threshold for the second stage would have to be selected so low that in less serious crash events, a decision to ignite to the second stage would be made very early measured as the result of the occupant's forward displacement. Because the second stage could also still have been ignited at a later point in time in order to bring the air bag into a protective condition, time is wasted which could be utilized for observing and evaluating the acceleration signal.
Particularly in the event of crashes of medium severity, a soft inflation of the air bag is desirable in order to reduce the stress to the occupant by an inflating air bag. This requires a controlling of the time difference between the first and the second stage which is a relevant parameter for influencing the inflating action of the air bag. The second stage must not be ignited too early but also not too late. On one hand, the limited modular stability of the gas generator requires that a minimum time period must be maintained between the ignition of the first and the second stage of the gas generator. On the other hand, however, an ignition must be prevented when the occupant's forward displacement has progressed so far that an ignition of the second stage may result in an injury.
The precise control of the time difference cannot be satisfactorily carried out for real crash sequences by the above-described known method with the staggered triggering threshold for the ignition of the first and second stage.
DE 27 45 620 A1 involves a method in which the evaluation process on which the triggering decision for the triggering of the second stage of the gas generator is based consists of the fact that the acceleration signal is compared with a threshold value. When the threshold value is exceeded, the triggering decision is made. However, a delay element causes the triggering of the second stage to be released only with the elapse of a defined time period from the ignition of the first stage. As a result, independently of the time sequence of the deceleration increase occurring in the event of an accident, the optimal time interval must be ensured between the triggering of the first and the second stage of the two-stage air bag system. The optimal time interval results from the demand that the optimal sound pressure occurring during the triggering is to be reduced. This method has the disadvantage that, in crash events which exhibit a very strong initial deceleration increase, the triggering decision to the second stage is made very early with the threshold being exceeded. The remaining time to the delayed triggering therefore elapses unutilized, which affects the evaluation concerning the seriousness of the accident. An evaluation of the entire course of the acceleration signal during the entire defined time period to the triggering could improve the evaluation with respect to the seriousness of the accident and thus the adaptation to the accident event.
WO95/19278 describes a multi-stage air bag system in which the various gas generators are triggered according to a triggering profile which, as a function of parameters which relate to the crash event and/or the occupant, determines the sequence and time sequence of the triggering of the gas generators. As a result, the inflating action of the multi-stage air bag system can be better adapted with respect to its restraining effect.